solve word problem.

Question

after the release of radioactive material into the atmosphere from a nuclear power plant at chernobyl in 1986, the hay in austria was contaminated by iodine 131.  If is is safe to feed teh cows when 10% of the iodine 131 remains, how long did the farmers need to wait to use this hay?

Answer 1

Half-life of I-131 is 8.02 days. If we start with x grams of I-131 then after 8.02 days it will have a mass of x/2. After a further 8.02 days it will have a mass of x/4. 0.5ⁿ=0.1 when n=log0.1/log0.5=3.32 approx. So after 3.32×8.02=26.64 days we have x/10 grams, that is, 10% of the original mass. If this is the acceptable safe level, then it takes about 26.6 days to reach the level.